Electronic devices such as computers, personal digital assistants, monitors, portable DVD players, and portable music players such as MP3 players typically have multiple power states. Two exemplary power states are “on” when the device is operating at full power and “off” when the device is turned off and uses very little or no power. Another exemplary power state is “sleep” when the device is turned on but uses less power than when in the “on” state, typically because one or more features of the device are disabled or suspended. Yet another exemplary power state is “hibernate” when the device's state is saved to non-volatile storage (typically the system's hard drive) and then the device is turned off. Sleep or hibernate states are typically used to reduce energy consumption, save battery life and enable the device to return to the “on” state more quickly than from the “off” state.
FIG. 1 is a perspective view of a computer system according to the prior art. A user may interact with the computer 100 and/or the display 105 using an input device, such as a keyboard 110 or a mouse 115. A button 120 may be used to turn on the computer 100 or the display 105. A light emitting diode (“LED”) 125 may be used as a status indicator to provide information to a user regarding a current power state of the computer 100 or the display 105, and optionally other operational information, such as diagnostic codes. When the computer 100 or the display 105 is turned on, the LED 125 emits light that is seen by the user. When the computer 100 enters the sleep state, the LED 125 pulses to alert the user the computer is in the sleep state. Other prior art systems may include more complex LED behavior. For example, some prior art systems having a built-in display activate the LED only if the computer is on and the display is off. Yet other prior art systems lacking an integrated display may turn on the LED whenever the computer is turned on. It should be understood that the foregoing descriptions are a general overview only as opposed to an exact or limiting statement of the prior art.
Alternatively, the LED may be combined with button 120 made of a transparent material that covers or overlays the LED. The light emitted by the LED is transmitted through the button and is seen by the user.
The perceived brightness of the LED 125 depends on the contrast between (1) the ambient light reflecting off the area surrounding the LED and (2) the light emanating directly from the LED, due to the way the human eye functions. The human eye registers differences in contrast rather than absolutes. Thus, for example, a light that has an unchanging absolute brightness appears much brighter in a dark room than outdoors on a sunny day. Accordingly, the way the eye perceives the brightness of the LED is by its contrast relative to the ambient light reflected off the area surrounding the LED. In some environments, such as dark rooms, the light emitted by the LED can be distracting or disruptive to the user. Prior art has developed means of sensing the ambient light level and adjusting the LED's luminance in order to maintain a constant perceived brightness (i.e., constant contrast) as the ambient light changes. Prior art has also achieved partial success in controlling the rate at which the LED's luminance changes so that the user perceives an approximately linear rate of change in brightness regardless of the ambient light level. What is needed are improved methods of controlling the brightness of the LED when it is changing so that the user perceives smoother changes in the brightness of the LED to provide a more pleasing visual effect under a variety of ambient lighting conditions.